One known type of angular rate sensor includes two suspended masses, each having one set of floating comb finger electrodes at its left-side and its right-side positions (left-side floating comb finger electrodes and right-side floating comb finger electrodes). Two sets of fixed comb finger electrodes are also provided, that is left-side fixed comb finger electrodes and right-side fixed comb finger electrodes. Each set of the fixed comb finger electrodes is interdigitated, in a parallel manner with a corresponding set of the floating comb electrodes so as not to contact the latter. The mass is vibrated in the x-direction by applying voltages alternately between the left-side floating comb finger electrodes and the left-side fixed comb finger electrodes, and between the right-side floating comb finger electrodes and the right-side fixed comb finger electrodes. When an angular rate of rotation about the z-axis is applied to the mass in a driving mode (which the masses are vibrating in an anti-phase manner), the mass receives a Coriolis force. The mass thus comes to vibrate also in the y-direction (elliptical vibration). Where the mass is a conductor or is formed by joining electrodes and a detection electrode is provided on a substrate so as to be parallel with the xz-plane of the mass, the capacitance between the detection electrode and the mass vibrates or varies so as to correspond to the y-component (angular rate component) of the elliptical vibration. A magnitude of the angular rate may be determined by measuring the vibration or variation of the capacitance. Reference is made to Japanese Patent Laid Open No. Hei. 5-248872 and Japanese Patent Application Nos. Hei. 8-152327 and Hei. 9.127148 which disclose sensors along the lines described above.
U.S. Pat. No. 5,635,638 discloses a micro-machined device having two suspended masses positioned near each other, with each of the masses being dithered along a dither axis. Two couplings, each including an arcuate member and an anchored support beam, are provided between the masses to allow relative anti-phase movement and resist relative in-phase movement. The coupling extends around a region intermediate the masses where a dither detection device is disposed.
In FIG. 4 of U.S. Pat. No. 5,635,638, the micro-machined sensor has a pair of masses, a plurality of beams connecting each of the masses and supporting the masses in a floating manner at an anchor portion. If an external force operates on the beam portions when the masses (vibrators) are being driven in an anti-phase manner, the beam portions supporting the masses on the substrate receive compression and tensile stress. As a result, a resonant frequency changes according to temperature changes, the vibration characteristics of the sensor possess hysteresis and are non-linear, the accuracy of the sensor is decreased. For example, in the above-mentioned sensor having a plurality of anchors, as a distance exists between the anchors, a vibration on the driving side affects a vibration of the detecting side which detects the angular rate signal when an angular rate operates on the masses. The accuracy of the sensor is thus decreased.
U.S. Pat. No. 5,349,855, which corresponds to Japanese Patent Laid Open No. Hei. 7-218268, discloses an inertia rate sensor. The sensor has a rotational mass disposed on a silicon substrate, with the rotational mass is supported by two support beams extending in the x-direction. The vibrating mass is vibrated by a pair of driving electrodes disposed on both sides in the y-direction of the vibrating mass. Two pair of detecting electrodes are disposed on the substrate to detect the vibration about the x-direction when an angular rate operates the rotating mass in a driving mode. In this sensor, a non-movable point (a point which does not receives stress in the x, y, z directions) in the driving mode (a condition in which the mass is vibrating in the anti-phase manner) exists, but a non-movable point (a point which does not receives stress in the x, y, z directions) in the detecting mode (a condition occurring by the Coliolis force when the mass is vibrating in the anti-phase manner) does not exit. This thus causes decreasing accuracy of the sensor when a vibration leak occurs by the sensor structure, and an effect by an external force operates on the sensor.
For example, when the angular rate operates the mass about the z-direction, a Coriolis force is exerted on the mass as follows. EQU F=2 m v .OMEGA.
where:
F: Coliolis force PA1 m: mass; PA1 v: speed of the mass; and PA1 .OMEGA.: angular rate
If the sensor has a vibration component decreasing the vibration by the Coliolis force in the driving mode, the accuracy of angular rate detecting signal may be inferior or affected even if an angular rate operates the sensor. The amplitude of the conventional vibration of the mass often becomes unstable when the vibration differs from the normal vibration direction (anti-phase manner).
In the angular sensor of U.S. Pat. No. 5,635,638, if a driving force operating the mass is not balanced by a dimensional fluctuation during manufacture, the vibration of the mass becomes unbalanced and non-linear vibration occurs. That is, by the unbalance of sift vibration of the resonant frequency, as unbalance fluctuation occurs, signal-to-noise ratio (SN ratio) of the angular rate will be inferior. As the mass of the driving mass corresponds to the mass of the detecting mass, if vibration in the detecting direction by the manufacturing dimensional fluctuation is generated in the driving mode, it causes the signal-to-noise ratio of angular rate signal to decrease. As the vibration in the driving mode leaks outward passing through the substrate, a vibration component reflected outward comes back into the substrate and its vibration is added to the usual vibration component, thus causing the signal-to-noise ratio of the angular rate signal to decrease. Also, as the driving signal vibrating the mass is transmitted to the detecting side, it causes the signal-to-noise ratio of the angular rate signal to decrease.
In light of the foregoing, a need exists for an angular rate sensor that is not susceptible to the same disadvantages and drawbacks as other known sensors.